For many years wire manufacturers have been applying coatings to both electrically insulate and mechanically protect their products. Such insulation is of particular importance to the manufacturers of magnet wire due to the nature of the final product made using this wire, such as electrical transformers.
These insulating processes generally require that the bare wire substrate be contacted with the wet enamel coating, thereby covering the wire with a layer of the coating, thereby, covering the wire with a layer of the coating material. The thickness of the coating and concentricity about the wire are then assured by passing the coated wire through some type of die arrangement which removes the excess coating and conforms the coating to a proper thickness and uniformity about the wire. The solvents, which carry the enamel solids are then driven off and the enamel cured by passing the coated wire through an oven.
These enamel coatings, when applied to the wire substrates, are in the form of low solid solutions. Generally, the solutions contain about 10 to 25 percent by weight of the enamel solids in organic solvents.
The particular method of applying the enamel coating to the wire substrate offers a number of problems for enamel coating designers. Since the enamel coating solutions contain low solids it requires a number of passes through an enamel applicator to build the required coating to the desired thickness. These thicknesses may vary from about 1 mil or less to about 6 mils depending on wire diameter. Therefore, it would be advantageous to develop a coating with higher solids contents thereby permiting high build-ups with each pass of the wire through the die resulting in fewer passes to produce the desired build. It should be noted that not only would high solids enamels be more efficient, requiring fewer passes, but these solutions would require a lower concentration of organic solvents and therefore, would likely be lower in cost due to the fact that the organic solvents represent a major cost of these enamel solutions.
Another benefit from such enamel coatings would be in the form of environmental protection because with fewer passes of the wire through the enamel and less organic solvent being in the solution less of the organic solvents would be evaporated into the atmosphere during the enameling process.
However, the enamel coating designers are faced with a problem that when one increases the solids content of a given enamel the viscosity rises significantly. This makes application of the enamel to the wire substrate difficult to control to a uniform thickness and concentricity. In addition, any such enameling coating must be able to meet the industry standards and requirements for both electrical and physical properties of the finished wire products.
One compositions for high solids enamels is taught in U.S. Pat. No. 4,578,312 in which high molecular weight polyether are used to formulate an enamel. However, it has been found that such a resin system, once applied to the wire, undergoes crazing and is dielectrically unstable. In addition, due to the presence of all high molecular weight polymer, the ability to increase the solids is limited by the final viscosity resulting from these resins.
Therefore, what is needed in this art, is a wire enameling coating which has high solids content and yet a low viscosity at application temperatures which will allow for the production of a coated wire substrate having a uniform thickness and concentricity and meets all of the electrical and physical properties required of such wires in the industry and will reduce crazing and increase the dielectric stability of the final wire product.